One hardly imagines a major seaport as being covered with sagebrush, tumbleweeds and vineyards. But if it is more than 35 miles from the nearest ocean--well, could be . . .

In fact, it is. And it is not only a world port but a foreign trade zone too.

Continuing the catalogue of the unusual, its 1,350 acres make it larger than at least three Southland cities--Hermosa Beach, Lawndale and Artesia.

This "seaport" is the Ontario Commerce Center, which fills the two miles from the edge of Ontario International Airport eastward to the Devore (15) Freeway and, in the other direction, stretches from near the San Bernardino (10) Freeway southward to about a mile from the Pomona (60) Freeway.

The reason it can be called a seaport is that last April it was made a part of Long Beach Foreign Trade Zone No. 50. Sub-zones are a truck-bed plant in Long Beach and a shipbuilding yard in San Diego, but the Commerce Center was the zone's first expansion. And, according to the developer, the Commerce Center is the largest privately owned foreign trade zone in the United States.

The first phase of the center, 282 acres off the Devore Freeway between Jurupa Street and Airport Drive, sold out within a year and a half of its opening in 1983. In it, 2.3 million square feet of new commercial and industrial buildings are complete or under construction.

On Wednesday, the second phase will be officially opened by the developer, Ontario Industrial Partners, consisting of John D. Lusk & Son, Cadillac Fairview/California Inc. and Shaw & Talbot Associated II. The latter is a joint venture of Ontario Land Associates, an affiliate of the Hillman Co., and California Partners, consisting of Donald Shaw and Milton Gottlieb.

In addition to opening the 290-acre Phase 2, the "grand gala," at which 1,000 guests are expected, will officially dedicate the center as a foreign trade zone and honor the developers who have built or are building in the first phase.

--Millikin Business Center by California Partners, Wilma Pacific and Northwestern Mutual Life Insurance Co., 28 acres with a 70,000-square-foot building as its first phase.

Guest will also see:

--Airport Tech Center by JS Development, a 75,000-square-foot complex of six multitenant buildings.

--The first of two developments by Ronald Binder, a 252,000-square-foot building on 10 acres; he plans another similar building.

--The 8.3-acre site of Leed Ontario II by Leed Properties, a complex where construction will start soon on buildings of 89,000 and 85,320 square feet.

--Domino's Pizza's completed and occupied regional headquarters, a 40,000-square-foot building on four acres, and Vanier Graphics, a 110,000-square-foot building on eight acres.

Sites will also be visited where facilities will soon be built for L. D. Brinkman and Laminations West and new developments will be started by Vogel Properties, Glaser Development, Jemco Properties and Sydney Investment Co.

Also being developed, in cooperation with the Ontario Redevelopment Agency, is a 100-acre auto center east of the Devore Freeway at Jurupa Avenue. Ten land parcels will be offered to dealers, with financial assistance available from the redevelopment agency; two dealers have already signed for land and four more are negotiating.

David W. Ariss, managing director of the center, said future phases will include office buildings, commercial properties, hotels, restaurants and service businesses, and the center will provide jobs for 35,000 people.

"The area already has a working population," he said, "and it is natural that industry will follow to take advantage of the established labor pool here." Ariss also said that, with the first phase well on its way, he and Coldwell Banker, exclusive sales agent, will concentrate their efforts on Phase 2, which has 43 land parcels ranging from three to 42 acres. A sales center is open daily from 8:30 a.m. to 5 p.m. at Jurupa and Rockefeller avenues.

Ariss said land costs in the Ontario Commerce Center average $3.50 a square foot, which he contrasted to $7 to $15 a square foot the same distance from John Wayne Airport in Orange County, or $25 to $30 a square foot near Los Angeles International Airport.

He pointed out that the benefits of the foreign trade zone are available to any company or individual leasing or buying in the center, yet the center's status as part of the zone does not increase costs in any way.

(A foreign trade zone, according to a spokesman for the Port of Long Beach, is basically a duty-free area. As one example of how it works, goods in transit through this country but not destined to remain here, may be warehoused in bond without paying duty.)

"It's a plus if you're in international trade in any way, even as just a part of your business, but it has no effect if you're solely a U. S. operation. Of course, over time it could make land or buildings in the center more valuable," he said.